1. Field of the Invention
The present invention is concerned with the recovery and recycle of molybdenum catalyst values from distillation residues obtained in the process of epoxidizing olefinic compounds with organic hydroperoxides in the presence of liquid solutions of dissolved molybdenum. This invention is particularly concerned with evaporative techniques for concentrating such molybdenum values.
2. Description of the Prior Art
Oxirane compounds such as ethylene oxide, propylene oxide, and their higher homologs are valuable articles of commerce. One of the most attractive processes for synthesis of those oxirane compounds is described by Kollar in U.S. Pat. No. 3,351,635. According to Kollar, the oxirane compound (e.g., propylene oxide) may be prepared by epoxidation of an olefinically unsaturated compound (e.g., propylene) by use of an organic hydroperoxide and a suitable catalyst such as molybdenum.
During the epoxidation reaction the hydroperoxide is converted almost quantitatively to the corresponding alcohol. That alcohol may be recovered as a coproduct with the oxirane compound. However, it is the oxirane which is of primary concern.
Kollar teaches that oxirane compounds may be prepared from a wide variety of olefins. Lower olefins having three or four carbon atoms in an aliphatic chain are advantageously epoxidized by the process. The class of olefins commonly termed alpha olefins or primary olefins are epoxidized in a particularly efficient manner by the process. It is known to those in the art that primary olefins, e.g., propylene, butene-1, decene-1, hexadecene-1 etc., are much more difficulty epoxidized than other forms of olefins, excluding only ethylene. Other forms of olefins which are much more easily epoxidized are substituted olefins, alkenes with internal unsaturation, cycloalkenes and the like. Kollar teaches that notwithstanding the relative difficulty in epoxidizing primary olefins, epoxidation proceeds more efficiently when molybdenum, titanium or tungsten catalysts are used. Molybdenum is of special interest. Kollar teaches that activity of those metals for epoxidation of the primary olefins is surprisingly high and can lead to high selectivity of propylene to propylene oxide. These high selectivities are obtained at high conversions of hydroperoxide (50% or higher) which conversion levels are important for commercial utilization of the technology.
Kollar's epoxidation reaction proceeds under pressure in the liquid state and, accordingly, a liquid solution of the metal catalyst is preferred. Preparation of suitable catalysts is taught in U.S. Pat. Nos. 3,434,975; 3,453,218; and 3,480,563. It has been found that such suitable catalysts comprise high molecular weight, highly complex molybdenum compounds which, because of their low volatility, are carried through the process steps used to recover and separate unreacted olefin, the alkylene oxide product and the by-product alcohol resulting from the reduction of the organic hydroperoxide.
When an olefin is epoxidized with an organic hydroperoxide in the presence of molybdenum-containing catalyst according to the Kollar process, a product mixture containing unreacted olefin, alkylene oxide, an alcohol corresponding to the organic hydroperoxide and molybdenum catalyst is obtained. This product mixture is subjected to a series of distillations in order to resolve it into separate streams comprising unreacted olefin, which may be recycled to the epoxidation zone, and substantially pure alkylene oxide and alcohol products. The distillation residue (hereafter "TBA bottoms") contains spent molybdenum catalyst, some alcohol, acids as well as high boiling organic residues.
After the separation of valuable products of the reaction, it is desirable (both from economic and ecologic standpoints) that the catalyst be recovered from the remaining organic distillation residue. This residue contains substantially all of the catalyst withdrawn in the product mixture from the epoxidation zone. The residue stream (which usually contains from about 0.1 to about 1.0 wt. % molybdenum) can be recycled directly to the epoxidation zone (as suggested in the Kollar patent cited above), but direct recycle results in a buildup within the system of impurities (e.g., acids) which are deleterious to subsequent epoxidation reactions. Moreover, disposal of the residue stream (or a portion thereof, as in the case of a purge stream from direct recycle) presents substantial pollution problems. Accordingly, various methods have been proposed for the removal and recovery of molybdenum values from the distillation residue.
It is known that molybdenum catalyst may be recovered from the distillation residue by extraction with various aqueous mediums. For example, Khuri (U.S. Pat. No. 3,763,303) suggests liquid/liquid extraction of residue with an extractant consisting essentially of water, recovering the molybdenum-containing aqueous extract, evaporating the extract, calcining the solid obtained in the extract evaporation step, and recovering molybdenum values as molybdenum trioxide. The molybdenum trioxide is insoluble in epoxidation zone reaction mediums and, accordingly, must be redissolved prior to reuse as an epoxidation catalyst. Khuri also teaches use of acids or bases as the extracting agent, converting molybdenum into a recoverable compound of the acid or base.
British Patent Specification No. 1,317,480 discloses a recovery process involving extraction of spent epoxidation catalyst solutions with water or aqueous ammonia and recovery of molybdenum values from the aqueous extract either by precipitating molybdenum therefrom as a phosphomolybdate or by distillative stripping of volatile organics and water.
Commonly-assigned U.S. patent application Ser. No. 226,967, filed Jan. 21, 1981, teaches a further improved extraction method for the recovery of molybdenum which method comprises extraction of distillation residues (e.g., TBA bottoms) with water (without an added acid or base) and a water-immiscible organic solvent for the organic residue. The aqueous extract solution phase which is obtained contains mostly water but also contains dissolved molybdenum values and may contain low molecular weight organic material from the "spent catalyst solution". Higher molecular weight organic materials remain in the organic extract phase.
Commonly-assigned U.S. patent application Ser. No. 226,969, filed Jan. 21, 1981, teaches that when distillation residues (e.g., TBA bottoms) are subjected to aqueous extraction as taught by either the British patent (supra) or U.S. patent application Ser. No. 226,967, active molybdenum catalyst may be prepared from the extract by stripping off water and lower molecular weight organic materials. The '969 application further teaches that the product obtained by wiped film evaporation of distillation residues is a suitable material for use in the extraction/reuse procedure disclosed therein.
Wiped film evaporation of distillation residues is disclosed in Levine (U.S. Pat. No. 3,819,663). According to Levine, customary and regular procedures cannot be employed to resolve this material in view of the tendency of the molybdenum-containing residue to cake, coat and block conventional apparatus. Levine further teaches that wiped film evaporation removes volatile materials including most of the acidic compounds which are deleterious in the epoxidation zone. Hence, the wiped film evaporation residue may desirably be recycled to the epoxidation zone to satisfy catalyst requirements. In one embodiment of the Levine method, distillation residue is rapidly heated to a bottoms temperature of 375.degree.-450.degree. F. (190.degree.-232.degree. C.) at essentially atmospheric pressure in an agitated (or wiped) film evaporator to separate about 60 to 85 wt. % of the charge as vapor. The residue resulting may be reused as a liquid in an epoxidation reaction or, in another embodiment, may be further concentrated in a second agitated film evaporator having a scraping means to recover a solid residue. The solids obtained in this latter embodiment may be redissolved and reused in an epoxidation reaction. In addition to providing a readily reusable catalyst stream, the method of Levine produces volatile organic materials which are essentially free of metal residues and thus can be employed directly in furnaces as a non-fouling fuel.
Wiped (or agitated) film evaporation is a modified form of falling film evaporation employing a heating surface consisting of one relatively large diameter, jacketed tube containing an internal agitator. Feed enters at the top of the jacketed section and is spread out into a thin, highly turbulent film by the blades of the agitator. Concentrate is removed from the bottom of the jacketed section. Vapors are generally removed at the top of the unit. The technique is particularly suited for viscous heat-sensitive materials, providing very small hold-up volumes and residence times to reach desired end concentrations. Disadvantages of the technique are high cost, internal moving parts in the apparatus which may require considerable maintenance, and the small capacity of individual units.
Thus, while Levine represents an important advance with respect to molybdenum recovery from distillation residues obtained in the process of epoxidizing olefinic compounds with organic hydroperoxides in the presence of liquid solutions of dissolved molybdenum, further improvements in the evaporative concentration technique are desirable. A particular problem addressed by the present invention is the capacity restrictions imposed by the wiped film evaporators of Levine.
Thus, an object of this invention is to provide a method for the recovery of molybdenum from such distillation residues. More specifically, an object of this invention is an improved evaporative method which may be used alone, or in combination with other known methods for the said recovery of molybdenum.
Another object of this invention is to provide a method whereby molybdenum is recovered as an active, high quality catalyst for the hydroperoxide oxidation of olefins.